This invention relates to the processing of AM-MSK encoded radio signals, and more particularly relates to the conversion of such signals to MSK form.
Amplitude modulated minimum shift keying has been used in commercial airline radios to carry data at 2400 bits/sec for the ACARS net. This coding imposes frequency offset minimum shift keying (MSK) modulation which can be represented by real numbers at baseband frequencies on the envelope of an amplitude modulated (AM) carrier. The format for encoding MSK on AM was specifically developed for use with analog AM radios employing envelope detectors. One advantage of AM-MSK coding is the simplicity of demodulation. However, the demodulation recommended in current Federal Aeronautics Administration (FAA) descriptions is not optimal, because it does not provide ideal matched filtering. In addition, for a message length of several seconds, Doppler frequency shifts due to relative motion of vehicles carrying the radio and the ground station may interfere with the decoding of the symbols transmitted with AM-MSK coding. This invention addresses these problems and provides a solution.
Techniques for modulating a carrier signal to provide MSK encoded signals are described in U.S. Pat. No. 5,834,986 (Myers, issued Nov. 10, 1998), but this patent does not describe in detail the decoding of such signals. A synchronous receiver for minimum shift keying transmission is described in U.S. Pat. No. 4,942,592 (Leitch et al., issued Jul. 17, 1990). A demodulator for demodulating digital signals modulated by minimum shift keying is described in U.S. Pat. No. 5,170,131 (Takahiro et al., issued Dec. 8, 1992). None of these patents address the problems solved by the present invention.